1. Field of the Invention
The present invention relates to self ballasted fluorescent lamps and to lighting fixtures using such lamps.
2. Description of Related Art
Self ballasted fluorescent lamps having fluorescent arc tubes are known. Such self ballasted fluorescent lamps generally have a ballast power supply and an E form base for screwing into a conventional incandescent light socket. The ballast power supply utilizes commercial AC power to provide the high frequency power needed to drive a fluorescent arc tube.
There are operational and safety hazards associated with the operation of fluorescent arc tubes when they reach the end of their useful lives. Near the end of a tube's life, it becomes difficult to operate the tube even when applying the rated voltage. As the tube ages, its electrodes no longer emit as well. The effective resistance of the tube increases. The current being forced through the tube causes the voltage drop across the tube to rise and the electrodes and tube overheat, sometimes to a very dangerous degree. There is a risk of fire and a risk that the tube will explode. Before any dangerous condition arises, it is desirable to stop operation of the ballast power supply and end operation of the tube.
Japanese Patent Laid-open No. 10-189279 discloses a ballast power supply driven fluorescent tube arrangement. The ballast power supply includes a switch that self-destructs at the end of tube life when the current flowing through the switch exceeds a predetermined amount. The switch opening stops the power supply. However, stopping the lamp is not reliable based solely on the current flow through the switch, in part because of wide variation in the electrical characteristics of the switch.